IntroductionAnalytical science, currently applied in support of a justice system, can establish a degree of resemblance between one substance and another by means of identifying the constituent elements, cations, anions, functional groups and by elucidating structure. Should two substances correspond, it may be concluded that they are chemically the same substance.

However, it has always been a defence that although the two substances in question are chemically the same, they have a different source. The analytical technique, stable isotope ratio mass spectrometry (IRMS), now allows this contention to be tested and potentially provides resolution.

Initial research has confirmed the potential probative power of the data provided by this analytical technique. The stable isotope compositions of elements which are part of a substance, are a function of the origin and history of that substance. That is, two substances which are chemically the same may have different stable isotope compositions if either their origin and/or history differ. This could remove the defence of the same substance being from different sources and thus be a significant advance in forensic science, crime detection and reduction.

This technique would significantly increase the probative power of analytical results for drugs, explosives, fibres, textiles, glass, paints, papers, inks, plastics, adhesives and materials in general, indeed any non-genetic trace.

Stable isotopes

Isotopes are defined as atoms whose nuclei contain the same number of protons, but a different number of neutrons. All but 12 elements exist as mixtures of isotopes. The proportions of these isotopes can vary greatly. An example of the stable isotopes of hydrogen, hydrogen-1 and hydrogen-2 (known as deuterium), is shown below:

The proportions of stable isotopes listed below are approximately the mean values found naturally throughout Earth. The small variations in these values are the way of characterising materials. The units in the following table are atom % (the particular concentration of that isotope).